The present invention relates to a self-diagnosis apparatus which performs the self-diagnosis for a fuel evaporative emission control system without discharging the fuel vapors, which have been generated in a fuel tank, into the atmosphere, and a method employing the same. In particular, the present invention is designed in such a way as to be capable of being used as a self-diagnosis apparatus, for use in a fuel evaporative emission control system, in which the fuel vapors, which have been generated in a fuel tank of a vehicle, are absorbed in a canister and then the fuel vapors thus absorbed in the canister are introduced into an intake manifold to be burned in an engine, and a method employing the same.
One example of the so-called fuel evaporative emission control system in which in order to prevent the fuel vapors, which have been generated in a fuel tank, from being discharged into the atmosphere, the fuel vapors are absorbed in an activated charcoal filled in a canister and then the fuel vapors thus absorbed in the canister are introduced through an intake manifold into an internal combustion engine to be burned therein is disclosed in Japanese patent application un-examined publication No. JP-A-63-85237.
However, if the vapor leaks are present in a passage between the fuel tank and the canister when introducing the fuel vapors into the canister, a part of the fuel vapors will not be introduced into the canister but be discharged into the atmosphere.
Therefore, there is need for judging whether or not the abnormal state such as the vapor leak is present in the passage between the fuel tank and the canister in such a fuel evaporative emission control system. As for a technology for detecting the vapor leak, there is the technology disclosed in U.S. Pat. No. 5,193,512 issued to Steinbrenner et al. on Mar. 16, 1993.
In the invention disclosed in U.S. Pat. No. 5,193,512, the internal pressure in the fuel tank is compared with a predetermined reference pressure, and the abnormality in the device for processing fuel vapors is detected in accordance with the comparison result. Then, in the case where the abnormality in that device is detected, the alarm alerts an operator so that the operator is able to perform the suitable processing, so that the leakage of the fuel vapors is minimized.
However, in the judgement of the abnormality in the fuel evaporative emission control system disclosed in U.S. Pat. No. 5,193,512, the internal pressure in the fuel tank is first detected, and then on the basis of the detection result, it is judged whether or not the fuel vapor processing device is in an abnormal state. Therefore, there is need for correcting the dispersion in the characteristics such as the accuracy, the temperature characteristics and the secular change of the pressure sensor provided in the fuel tank for detecting the internal pressure in the fuel tank, thereby to perform the detection.
As examples of the technology for detecting the internal pressure in the fuel tank by the pressure sensor to detect the abnormality of the fuel evaporative emission control system, there are the technologies disclosed in, for example, JP-A-2-102360, JP-A-4-308350 and JP-A-4-318268 as Japanese Publications of Unexamined Patent Applications which were filed in the Japanese Patent Office.